Known techniques for introducing an impurity into the surface of a solid material include a plasma doping (PD) method of introducing an impurity into the solid material with a low energy after ionizing the impurity (see, for example, Patent Document 1).
The ion implantation method is currently the most widespread method for introducing an impurity. The plasma doping method is mentioned as an item in Non-Patent Document 1, and is stated in Non-Patent Document 2 as being an impurity-introducing technique of the next generation which should replace the ion implantation method.
Ion implantation employs an apparatus configuration including an ion source for generating a plasma from a gas, an analyzing magnet for mass separation to select ions of interest from among ions extracted from the ion source, an electrode for accelerating the ions of interest, and a process chamber for implanting the accelerated ions of interest into a silicon substrate. With ion implantation, the impurity can be implanted to a shallow depth by using a small energy and a small acceleration energy for extracting ions from the ion source. However, a decrease in the extraction energy reduces the number of ions to be extracted. Moreover, if the acceleration energy is decreased, the diameter of the ion beam being delivered from the ion source onto the wafer is widened by the repulsive force acting between ions due to their electric charge, whereby the beam line hits the inner wall of the chamber, thus losing a large number of ions and thereby lowering the implantation throughput. In a case where B+ ions are implanted, for example, the throughput starts to decrease when the acceleration energy is 2 keV or less, and the delivery of the beam itself becomes difficult when the acceleration energy is 0.5 keV or less. Moreover, even when the energy is lowered down to 0.5 keV, B is implanted to a depth of as much as about 20 nm. Thus, if one attempts to form an extension region thinner than this, the productivity will be very low.
In contrast, the plasma doping method employs an apparatus configuration including a cylindrical vacuum chamber capable of accommodating a silicon substrate therein, a plasma generation source for inducing a plasma, a bias electrode on which the silicon substrate is mounted, and a bias power source for adjusting the potential of the bias electrode. Thus, the plasma doping method employs an apparatus configuration that is totally different from that of ion implantation, in which neither the analyzing magnet nor the acceleration electrode is provided. Specifically, a bias electrode serving also as a wafer holder is provided in a vacuum chamber, and ions in the plasma are accelerated and introduced into the wafer by the potential generated between the plasma and the wafer. Thus, an impurity can be introduced by directly using a low-energy plasma, whereby the wafer can be irradiated with a large amount of low-energy ions as compared with ion implantation. Specifically, the dose rate in plasma doping is an order or orders of magnitude greater than that of ion implantation, and it is possible with this characteristic to maintain a high throughput even with low energy B implantation.
In addition, the present inventors have developed a process technique for forming an extension region that is very shallow and has a low resistance, based on the plasma doping method (Non-Patent Document 3).
Moreover, the present inventors have proposed a method with which it is possible to stably generate and sustain a plasma and to easily control the amount of dopant implantation, while enhancing the level of safety by diluting B2H6, which is toxic and highly hazardous to humans, as much as possible and without decreasing the doping efficiency (Patent Document 2). With this method, the B2H6 gas as a substance containing the dopant impurity is diluted with an He gas having a low ionization energy, and the He plasma is generated in advance, after which B2H6 is discharged. Moreover, the present inventors have also proposed that the concentration of the B2H6 gas in this method is preferably less than 0.05% by mass.
Furthermore, the present inventors have proposed a plasma doping method in which the doping time and the concentration of the gas containing an impurity are set so that the dose is constant without time dependency to enhance the dose control precision (Patent Document 3). Specifically, the present inventors have discovered that when a bias is applied while the silicon substrate is irradiated with a B2H6/He plasma, for example, there is a time period over which the dose of boron is substantially constant, and Patent Document 3 discloses a method for controlling the dose by using the time period, over which the dose stays substantially constant over time, as the process window.
Patent Document 1: U.S. Pat. No. 4,912,065
Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-179592
Patent Document 3: WO06/064772
Non-Patent Document 1: International Technology Roadmap for Semiconductors 2001 Edition (ITRS 2001) (Particularly, Shallow Junction Ion Doping in FIG. 30 of Front End Process)
Non-Patent Document 2: International Technology Roadmap for Semiconductors 2003 Edition (ITRS 2003)
Non-Patent Document 3: Y. Sasaki, et al., B2H6 Plasma Doping with “In-situ He Pre-amorphization”, Symp. on VLSI Tech., 2004, p. 180